The present invention relates to a combined cycle land-based power generating system in which the heat content of the exhaust gases from the gas turbine is recovered in a heat recovery steam generator for heating steam for expansion through the steam turbine, The invention particularly related to reheating spent cooling steam from the gas turbine and exhaust steam from the HP steam turbine outlet in the heat recovery steam generator for use in the IP steam turbine.
In prior combined cycle power generating systems, hot component parts of the gas turbine are cooled by introducing a fluid medium, for example, cooling steam, from the intermediate pressure section of the heat recovery steam generator supplemented by steam exhausting from the steam turbine into the gas turbine. Typically, cooling steam is provided from the intermediate pressure section of the heat recovery steam generator supplemented by a controlled portion of the steam from the exhaust from the HP section of the steam turbine only to the extent the steam is needed to cool the component parts of the gas turbine. The balance of the steam from the steam turbine HP section not needed for cooling purposes in the gas turbine is conventionally forwarded to the reheat section of the heat recovery steam generator for reheating. The energy for reheating the unneeded steam from the steam turbine is obtained from the exhaust gases of the gas turbine flowing through the heat recovery steam generator. The reheated steam is then typically combined with the spent cooling steam from the gas turbine for flow into the inlet of another portion of the steam turbine, e.g., the intermediate pressure (IP) steam turbine inlet.
The system described above is set forth in U.S. Pat. No. 5,428,950, of common assignee herewith. At that time, the cooling cycle duty and steam flow was believed sufficient to provide a mix of spent cooling steam from the gas turbine and the reheat steam at a temperature at or near the required inlet temperature for the IP steam turbine section for optimum performance. Increased steam cooling steam flow requirements of the gas turbine, however, have significantly and substantially reduced the temperature of the mix of spent cooling steam exiting the gas turbine and the reheat steam exiting the heat recovery steam generator to a temperature well below the optimum temperature of the steam supplied to the IP steam turbine inlet. With this reduced temperature of the mix, reduced performance of the combined cycle system results. It is this performance penalty which the present invention addresses.
In accordance with a preferred embodiment of the present invention, and in a combined cycle system where the temperature of the mix of spent cooling steam exiting the gas turbine is significantly lower than the optimum temperature of the steam at the IP steam turbine inlet, the spent cooling steam and exhaust steam from the HP steam turbine section are combined for flow through the reheat section of the heat recovery steam generator prior to delivery to the IP steam turbine inlet. It will be appreciated that the parallel flow arrangement of heat recovery in the aforementioned prior system disclosed in U.S. Pat. No. 5,428,950, i.e., (i) recovering heat from the exhaust gases of the gas turbine in the reheat section to reheat the exhaust steam from the HP section of the steam turbine, and (ii) generating heat from steam cooling the hot component parts of the gas turbine to provide heated spent cooling steam and combining the reheated exhaust steam and the spent cooling steam for flow to the IP section of the steam turbine is penalized by the greater of the pressure drops of the parallel flow paths. Consequently, if the steam temperature of the mix is comparable to or even slightly below the desired inlet temperature to the IP section of the steam turbine, the parallel arrangement is advantageous. However, as the cooling steam flow requirements increase for the gas turbine, the temperature of the mix of spent cooling steam and reheat steam becomes significantly and substantially lower than the desired input temperature to the IP steam turbine. In accordance with a preferred embodiment, a series arrangement for the reheat and spent cooling steam is provided to raise the temperature of the mix of steam provided to the IP section of the steam turbine. Moreover, a net increase in combined cycle performance is obtained by mixing the spent cooling steam and HP steam turbine exhaust for combined passage through the reheat section of the HRSG to substantially maintain the desired steam temperature at the intermediate pressure turbine inlet when the steam cooling flow requirements are high. The pressure loss caused by the combined pressure drops through the gas turbine and reheater, when the spent cooling steam and HP section exhaust steam are combined in series and pass through the reheater of the heat recovery steam generator, comprise the penalty for maintaining the mixed steam at the IP inlet of the steam turbine at the desired temperature. However, where the steam cooling requirements of the gas turbine are substantial and there is a consequential substantial reduction in temperature of the mixed cooling steam and reheat steam at the IP inlet of the steam turbine, the combined cycle system employing series reheat rather than the parallel reheat as set forth in the prior U.S. Pat. No. 5,428,950 affords improved overall performance.
In a preferred embodiment according to the present invention, there is provided in a combined cycle system including a gas turbine, a steam turbine and a heat recovery steam generator wherein gas turbine exhaust is used in the heat recovery steam generator for heating steam for the steam turbine, a method of operating the combined cycle system comprising the steps of supplying steam from the intermediate pressure section of the heat recovery steam generator to the gag turbine to cool component parts thereof, supplementing the steam from the intermediate pressure section of the heat recovery steam generator by supplying a first portion of steam from a high pressure section of the steam turbine to the gas turbine to cool component parts thereof, combining spent cooling steam from the gas turbine and a second portion of steam from the high pressure section of the steam turbine, reheating the combined spent cooling steam and the second steam portion in the heat recovery steam generator and flowing the reheated combined spent cooling steam and the second steam portion to an intermediate pressure section of the steam turbine.
In a further preferred embodiment according to the present invention, there is provided a method of operating a combined cycle system comprising the steps of supplying steam from an intermediate pressure section of a heat recovery steam generator to a gas turbine to cool component parts thereof, supplementing the steam from the intermediate pressure section of the heat recovery steam generator by supplying a first portion of cooling steam from an ancillary steam turbine to the gas turbine for cooling component parts thereof, combining a second portion of cooling steam from the ancillary turbine and spent cooling steam from the gas turbine for flow through the heat recovery generator heated by exhaust gases from the gas turbine and flowing the heated combined cooling steam to another section of the ancillary turbine.
In a still further preferred embodiment according to the present invention, there is provided a combined cycle system comprising a gas turbine, a steam turbine and a multi-pressure heat recovery steam generator wherein gas turbine exhaust gas is used in the heat recovery steam generator for reheating steam for the steam turbine, a supply passage for supplying gas turbine cooling duty steam from an intermediate pressure section of the heat recovery steam generator supplemented by a portion of the steam exhausting from a high pressure section of the steam turbine to the gas turbine for cooling turbine parts, a first passage in communication with the high pressure steam turbine section for supplying the supplemental steam portion to the supply passage, a reheater in the heat recovery steam generator, a second passage for flowing spent cooling steam from the gas turbine to the reheater, a third passage for flowing another portion of steam exhausted from the high pressure turbine section to the reheater, the spent cooling steam and exhaust steam being combined for flow through the reheater and a fourth passage in communication with the reheater and the intermediate pressure section of the steam turbine for flowing the reheated combined steam flows exiting the reheater to the intermediate pressure section of the steam turbine.